[TiddlyWeb] generate etags for filtered or searched results

[cdent]

If a filter or search returns a large dataset it may be useful to just return a 304 if we are able to generate satisfactory Etags for the content.

Because generating the Etags requires reading the stuff, this is not likely to be a savings in server side handling, but could be on the client side.

This is a maybe we should do, not a we should do.

[cdent]

This is already present, as a part of tiddlyweb/web/sendtiddlers. Any tiddler collection gets Etag.